Front ends of radio receivers often include gain-controlled amplifiers and automatic gain control (AGC) circuits. An AGC circuit keeps the output signal from the gain-controlled amplifier in a desired amplitude range regardless of input signal level. The desired amplitude range is typically one that allows subsequent signal processing to take place in the most effective manner. For example, when subsequent processing includes digital conversion, the output signal of the gain-controlled amplifier is often managed to use the full range of an analog-to-digital (A/D) converter to minimize the effects of quantization noise, avoid the need for a costly high-resolution A/D converter, and otherwise extend the dynamic range of the radio receiver.
While gain-controlled amplifiers provide significant benefits for receiver designs, they also pose more serious distortion problems than constant-gain amplifiers. Often, communication signals use phase to convey information. Hence, phase distortion can lead to errors in recovering the conveyed information. Often, communication signals use carriers of sufficient bandwidth so that intermodulation falls in the bandwidth of interest, either directly or through aliasing. Intermodulation can also lead to errors in recovering the conveyed information. Gain-controlled amplifiers suffer from these types of distortions more than constant-gain amplifiers.
Conventional gain-controlled amplifiers are often associated with filtering or tuning networks. Often, gain-controlled amplifiers, perhaps in conjunction with such networks, can be devised to minimize phase distortion by causing the amplifier to impart roughly equal amounts of phase shift to signals being amplified at different gains. Unfortunately, such techniques tend to cause intermodulation distortion to become unacceptably prominent. Conversely, conventional gain-controlled amplifier circuits may be devised which minimize the intermodulation distortion. However, these circuits usually suffer from a great degree of phase variance between low gain and high gain, leading to unacceptable phase distortion.
Too often, a radio receiver designer must select components and design filtering and other networks which compromise one type of distortion against the other to find a solution that just manages to work for a given application. This is an undesirable and inflexible approach that often leads to a great reduction in the population of components from which selections can be made. In a typical application, the conventional approach often leads to the use of difficult-to-obtain and expensive components. In many applications, performance suffers because the receiver front end introduces an excessive amount of distortion.